The spirit level is a familiar tool in which the position of an air bubble within a curved glass tube partially filled with a liquid indicates whether the tool is level or not. This principle has given rise to many different embodiments such as one disclosed in Scopacasa, U.S. Pat. No. 3,786,472. There, a toroid chamber is used which is half-filled with mercury. Pairs of electrodes are spaced around the periphery of the chamber. Contact between the mercury and variously situated electrodes as the chamber is inclined corresponds to predetermined angles of displacement. Cantarella U.S. Pat. No. 4,167,818 and Kent patent U.S. Pat. No. 4,672,753 both employ arcuate resistance elements which are variably submerged in a conductive liquid as the housing is tilted. The Cantarella patent applies a DC or AC potential between the resistance sectors and uses a bridge connected to circular hoop which is always in contact with the liquid to measure the change in resistance caused by the liquid wetting different parts of the sectors as the housing is tilted. Beitzer patent U.S. Pat. No. 4,536,967 provides a thermal shunt about the glass tube and accurate spacing and symmetry of the electrodes to minimize the effect of changes in fluid temperature on its resistivity.
Donati U.S. Pat. No. 4,937,518 uses methylene diglycol as a conductive liquid which partially fills a chamber formed between two spaced-apart ceramic plates. Each plate bears a pair of imprinted palladium electrodes in the shape of semi-circular segments arranged with their respective cordal edges facing each other and with their curved edges lying on the same imaginary circumference. The liquid forms a resistive electrical connection between the electrodes which varies in accordance with the position of the liquid.
The problem with the above described devices employing fluid electrolytes is that they tend to suffer from both leakage currents and electroplating of their sensing elements. Leakage currents are possible in certain embodiments because of surface adsorption of the electrolyte. Leakage current causes non-linearity, especially a thigh tilt angles. Asymmetry in the exciting AC voltage that is necessary in order to measure the change in resistance occasioned by movement of the sensing elements with respect to the liquid may cause electroplating of the sensing elements, as may accidental exposure to DC voltage. Electroplating of the sensing elements over time brings a battery effect into a tilt sensor and degrades the linearity of angular measurement and scale factor which, in some cases, can be noticeable even after a very short period of operation.
Some prior art devices have employed curved glass tubes in order to achieve a linear response to change in tilt angle. Such devices must be carefully fabricated to provide arcuate sectors which form portions of a perfect circle and are therefore quite costly. Those devices which employ mercury as the conductive liquid tend to be insensitive to small changes in angular rotation because of mercury's tendency to roll erratically and suffer also from the polluting effects of mercury's high vapor pressure.
On the other hand, those devices employing conventional electrolytes in ceramic instead of glass structures suffer, in addition to the battery effect, from surface absorption of the electrolyte by the porous ceramic surface which gives rise to surface leakage currents which degrade the linearity.